A Beckman XL-I analytical ultracentrifuge is requested. This state-of- the-art instrument will be available to the six major users and other members of the university. The XL-I will be used to characterize macromolecular interactions using both equilibrium and velocity studies. The XL-I will greatly enhance the ability of the major users to determine properties defining macromolecular interactions such as equilibrium constants, stoichiometrics and thermodynamic parameters. Such measurements will facilitate substantial progress and open new opportunities in current NIH-funded projects.